Handle single node in PlotLayer

[ackurth]

Currently, the following code plotting a single node with spatial metadata with internal plotting routines, fails with a Python level TypeError:

import nest
import matplotlib.pyplot as plt

num_neurons = 1
extent = 2

neuron = nest.Create(
    'iaf_psc_exp',
    num_neurons,
    positions=nest.spatial.free(
        pos=nest.random.uniform(min=-extent / 2, max=extent / 2),
        num_dimensions=2,
        edge_wrap=True)
)
nest.PlotLayer(neuron)

This PR tries to fix this problem.

[stinebuu]

Thanks!

[hakonsbm]

Although the single-node case results in a TypeError, I think it's more straight-forward to have something like

if (len(layer) == 1):
    pos = GetPosition(layer)
else:
    pos = zip(*GetPosition(layer))

or even as a single line

pos = GetPosition(layer) if len(layer) == 1 else zip(*GetPosition(layer))

[ackurth]

Although the single-node case results in a TypeError, I think it's more straight-forward to have something like

if (len(layer) == 1):
    pos = GetPosition(layer)
else:
    pos = zip(*GetPosition(layer))

or even as a single line

pos = GetPosition(layer) if len(layer) == 1 else zip(*GetPosition(layer))

I changed the code accordingly.

[hakonsbm]

Thanks!